Optical data storage systems that utilize a focused laser beam to record and instantaneously play back information are very attractive in the computer mass storage industry. Such optical data storage systems offer very high data rates with very high storage density and rapid random access to the data stored on the information medium, most commonly an optical disc. In these types of optical disc memory systems reading and writing data is often accomplished using a single laser source functioning at two respective intensities. During either operation, light from the laser source passes through an objective lens which converges the light beam to a specific focal point on the optical disc. During data retrieval, the laser light is focused on the recording medium and is altered by the information of the data storage medium. This light is then reflected off the disc, back through the objective lens, to a photodetector. It is this reflected signal that transmits the recorded information. It is thus especially important that, when information is being written to or read from the memory, the objective lens, and the exiting focused beam, be precisely focused at the center of the correct track so that the information may be accurately written and retrieved.
In order to attain a precise reading of the information stored on the disc, it is necessary to be able to move the objective lens in both a focusing (i.e., perpendicular to the plane of the disc) or Z direction in order to focus the laser beam to a small point of light on a precise location of the disc to write or retrieve information, and in a tracking (i.e., radial from the center of the disc) or Y direction to position the beam over the exact center of the desired information track on the disc. Focus and tracking corrections may be effected by moving the objective lens in either the direction of the optical axis of the lens for focusing, or in a direction perpendicular to the optical axis for tracking.
In these systems, the position of the objective lens in the focus and tracking directions is commonly adjusted by control systems. Actuators support the objective lens and convert position correction signals from the feedback control systems into movement of the objective lens. Most commonly, these actuators comprise moving coils, stationary magnets, and a stationary yoke, wherein a magnetic field is produced in an air gap between the yoke and magnets. U.S. Pat. No. 4,568,142 issued to Iguma and entitled "Objective Lens Driving Apparatus" illustrates an actuator of this type wherein the actuator includes rectangular magnets positioned within U-shaped yokes. The yokes are spaced from one another with their north poles opposing, in close enough proximity to one another to form a magnetic circuit. A square-shaped focusing coil is bonded to the outsides of a square-shaped lens frame. Four tracking coils are bonded on the corners of the focusing coil. The ends of the focusing coil are then positioned within the air gaps formed by each of the U-shaped yokes so that the focusing coil straddles the yokes. Because the focusing coil must extend around these "center" or "inner" yoke plates, the coil cannot be wound as tightly as desired and the rigidity of the coil construction is compromised. Further, in this type of closed magnetic circuit design, the majority of coil wire is positioned outside the air gaps, significantly reducing the efficiency of the actuator.
In most optical systems, the stiffness of the coil in the air gap has to be very high and the coil decoupling resonance frequency should be above 10 kHz, and is most desirably above 25 kHz. In many types of prior actuator designs, large amounts of coil wire in the magnetic air gap are often required to achieve maximum motor performance. To place such a large amount of coil within the air gap and still conform to the limited space constraints of the actuator design, the coil must be wholly or partially "freestanding", or must be wound on the thinnest bobbin possible. These types of coil configurations have low stiffness and typically decouple at lower frequencies. The dynamic resonance behavior of many actuator designs can also cause the coil to unwind during operation.
Other actuator designs have used the same magnetic air gap to develop focus and tracking motor forces such that the tracking coil(s) is glued onto the focus(s) coil or vice versa, in an attempt to save parts, space, and weight. In these types of designs, the decoupling frequency of the tracking coil(s) glued onto a freestanding focus coil is typically around 15 kHz, significantly below the preferred decoupling frequency.